Addition Cascade of Benzyl Cycloketone

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Radical C−C Bond Cleavage/Addition Cascade of Benzyl Cycloketone Oxime Ethers Enabled by Photogenerated Cyclic Iminyl Radicals Peng-Zi Wang,† Bin-Qing He,† Ying Cheng,† Jia-Rong Chen,*,† and Wen-Jing Xiao*,†,‡

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†

CCNU-uOttawa Joint Research Centre, Hubei International Scientific and Technological Cooperation Base of Pesticide and Green Synthesis, Key Laboratory of Pesticides & Chemical Biology Ministry of Education, College of Chemistry, Central China Normal University, 152 Luoyu Road, Wuhan, Hubei 430079, China ‡ State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China S Supporting Information *

ABSTRACT: A light-driven, metal-free, and iminyl radical-mediated ring-opening C−C bond cleavage/addition cascade of O4-methoxybenzyl oxime ethers and alkenes is described for the first time. The reaction shows a broad substrate scope and high functional group compatibility with both components, giving the corresponding valuable oxo nitriles in generally good yields. Key to the success of this protocol is the generation of cyclic iminyl radicals from the O-4-methoxybenzyl oxime ethers via a photocatalytic hydrogen atom transfer (HAT) process. The proposed main pathway is also supported by the preliminary mechanistic studies.

O

Despite being powerful, these visible light-driven SET strategies typically require the overlap in the redox potential range of the excited state photocatalysts and oxime esters to render the SET process thermodynamically feasible. In the case of oxime esters, the carboxylate anion formed after N−O bond scission often resulted in some competitive side reactions.12,13 As a result, it is both synthetically and mechanistically interesting to further explore other types of oxime precursors and new activation methods, which would allow their conversion to iminyl radicals in a more economical manner, and thus expand the reaction profile in order to access more structually diverse valuable nitriles.16 As a complementary method to SET, visible-light-driven hydrogen atom transfer (HAT) has frequently been used to activate substrates, without the limitations of redox potentials, enabling development of many otherwise inaccessible C−H activation/functionalization reactions.17 Despite some application of the iminyl radical in C−H bond activation by the HAT process,6 the HAT strategy has rarely been used toward iminyl radical generation. In 2000, Walton first disclosed that the ditert-butyl peroxide (DTBP)-derived t-BuO· radical could abstract the H atom attached to the carbon adjacent to the ether oxygen in O-alkyl arylaldoxime ethers, ArCHNOCHR2, to furnish oxyalkyl radicals;18 the resultant oxyalkyl radicals

ver the past deacades, nitrogen-centered radicals (NCRs) have been established as a versatile class of active species with wide application in various C−N bond formation.1 Notably, in recent years, the dramatic advances in visible light photoredox catalysis have significantly pushed the boundaries of nitrogen radical chemistry, and allowed chemists to unearth numerous synthetically valuable transformations,2 including NCR-mediated radical addition,3,4 cross-coupling,5 C−H bond activation,6 C−C bond cleavage,7 and even nitrogen radical catalysis.8 In this context, iminyl radicals have attracted considerable interest from the synthetic community. Such type of radical species allowed direct incorporation of an imine moiety and remote sp3 C−H bond activation. More recently, cyclic iminyl radicals have also been extensively explored in ringopening C−C bond cleavage and functionalization at the resultant cyanoalkyl radicals.9−13 Key to the success of these methods is the cyclic iminyl radical generation from the corresponding oxime precursors that is enabled by the cleavage of their N−O bonds. The majority of the known strategies for N−O bond cleavage of oximes falls into three different categories based on the activation modes (Scheme 1A): (1) homolytic C−N bond cleavage under somewhat harsh conditions, such as heating, UV, or microwave irradiation;9 (2) transition-metal-catalyzed SET-mediated N−O bond cleavage of redox-active oximes;10,11 (3) visible-light-driven SET-mediated N−O bond cleavage of redox-active oximes and their derivatives under mild conditions.12−15 © XXXX American Chemical Society

Received: July 20, 2019

A

DOI: 10.1021/acs.orglett.9b02535 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Condition Optimizationa

Scheme 1. Strategies for Generation of Iminyl Radicals and Reaction Design

underwent facile N−O bond cleavage to give iminyl radicals with release of aldehyde or ketone (R2O). Surprisingly, general application of this strategy in organic synthesis has yet been extensively explored. Building on this pioneering work and our recent studies on the iminyl radical chemistry,11,12 we speculated that application of the strategy of photocatalytic HAT to benzyl oxime ethers of 1 would provide a new access to cyclic iminyl radicals with release of aldehyde (Scheme 1B). If such a hypothesis works, a cyclic iminyl radical mediated cascade ringopening C−C bond cleavage/addition to electron-defficient alkenes might be achieved, which is a challenging transformation under our previous SET conditions.12 The feasibility of this working hypothesis was also corroborated by the recent study of Itoh, which demonstrated that scucessful iminyl radical generation from O-4-methoxybenzyl oxime ether can be achieved, when using 2-chloroanthraquinone (2-Cl-AQN) as the HAT catalyst under light irradiation. With this method, Itoh and co-workers developed an efficient pyrroline synthesis via intramolecular cyclization of iminyl radical to the pendant alkene moiety.19 Initially, we selected O-4-methoxybenzyl oxime ether 1a-I and 2-naphthyl acrylate 2a as the model substrates, and examined the feasibility of the reaction with commonly used photoredox catalysts (e.g., Ir[dF(CF3)ppy]dtbbpyPF6, 4CzIPN, Eosin Y) under 7 W blue LEDs irradiation.20 However, the reaction did not occur and both reactants reamain intact. Then, we screened a range of HAT catalysts, such as quinones, benzophenones, DDQ, acridone, and thioxanthone, with 2-butanone as solvent. Among them, quinone derivatives proved to be superior to others, and 2-chloroanthraquinone (2-Cl-AQN) was identified to be the best candidate, giving the desired product 3aa in 44% yield (Table 1, entry 1). With 2-Cl-AQN as catalyst, we continued to examine other parameters to improve the yields. A simple screen of bases revealed that the base had an important role on the reaction. Either lower yields or no product were observed when the reaction was performed in the presence of other base or without base (entries 2−5). The results of entries 6−8 demonstrated that 2-butanone was still the best of choice,

entry

oxime ether

base

solvent (x)

yield (%)b

1 2 3 4 5 6 7 8 9 10c 11c 12c 13c 14e 15f 16c,g 17c,g,h

1a-I 1a-I 1a-I 1a-I 1a-I 1a-I 1a-I 1a-I 1a-I 1a-I 1a-II 1a-III 1a-IV 1a-I 1a-I 1a-I 1a-I

K2CO3 Li2CO3 Cs2CO3 Na2CO3 − K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3

2-butanone (1.0) 2-butanone (1.0) 2-butanone (1.0) 2-butanone (1.0) 2-butanone (1.0) MeOH (1.0) THF (1.0) CYC (1.0) 2-butanone (4.0) 2-butanone (4.0) 2-butanone (4.0) 2-butanone (4.0) 2-butanone (4.0) 2-butanone (4.0) 2-butanone (4.0) 2-butanone (4.0) 2-butanone (4.0)

44 13 trace 32 22 trace 21 43 47 67 39d 21d 8d trace trace 71 (3ab) 70

a

Conditions: 1 (0.2 mmol), 2a (0.4 mmol), 2-Cl-AQN (0.02 mmol, 0.1 equiv), and base (0.4 mmol, 2.0 equiv) in solvent (x mL) at 25 °C under irradiation of 370 nm LEDs for 12 h. bIsolated yield. cUse of 0.5 equiv of 2-Cl-AQN. d1H NMR yield with 1,3,5-trimethoxybenzene as an internal standard. eWithout 2-Cl-AQN. fWithout light irradiation. gWith 2b (0.4 mmol). hUnder irradiation of 390 nm LEDs for 12. CYC = cyclohexanone.

while the cyclohexanone gave rise to a comparable yield (entry 8). Finally, when the reaction was performed with 0.5 equiv of 2Cl-AQN under more diluted conditions (0.05 M), a 67% yield of 3aa was obtained (entry 10). Notably, variation of the electronic properties of the phenyl group of 1a disclosed that, compared to 1a-I, substrates with a more electron-rich (1a-II, 1a-III) or neutral phenyl ring (1a-IV) resulted in an obvious decrease of yield (entries 11−13). In the control experiments, no product was observed in the absence of either photocatalyst or light (entries 14−15). In addition to 2a, 4-methoxyphenyl vinyl ketone 2b also reacted with 1a-I very well under the optimum conditions, giving product 3ab in 71% yield (entry 16). Notably, the reaction performed under visible light irradiation of 390 nm LEDs could give rise to a comparable yield (entry 17).20 With the optimal reaction conditions established, we first investigated the substrate scope of the radical C−C bond cleavage/addition cascade by reacting a set of cycloketone oxime ethers 1 with 4-methoxyphenyl vinyl ketone 2b on a 0.2 mmol scale. As highlighted in Scheme 2, the catalytic system showed broad substrate scope and high functional group tolerance. For instance, in addition to 1a-I, a range of monosubstituted Obenzyl oxime ethers (1b−i) bearing functionalities such as esters (1b−c), cyano (1d), ether (1e), amine (1f), phenyl (1g), and naphthyl (1h−i) groups at the 3-position reacted well with 2b, B

DOI: 10.1021/acs.orglett.9b02535 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 2. Scope of the Benzyl Cycloketone Oxime Ethersa,b

Scheme 3. Scope of the Alkenesa,b

a

Conditions: 1h (0.2 mmol), 2 (0.4 mmol, 2.0 equiv), 2-Cl-AQN (0.5 equiv), K2CO3 (0.4 mmol, 2.0 equiv), and 2-butanone (4.0 mL) under the irradiation of 370 nm LEDs. bIsolated yield. cThe d.r. was determined by HPLC analysis using a chiral stationary phase. dThe d.r. was determined by 1H NMR analysis of the product after flash chromatography.

current protocol could also be extended to allyl sulfone 2k, and the corresponding product 3hk was isolated in 64% yield. To obtain some insight into the mechanism, we first performed several control experiments with substrates 1a-I and 2e under the standard conditions. It was found that the target reaction was completely inhibited and no desired product 3ae was observed, when a stoichiometric amount of radical scavenger TEMPO or (PhSe)2 was added to the mixture (Scheme 4A). The corresponding radical trapping adducts 4 and 5 were detected respectively, which should be derived from cyanoalkyl radical 1a-A. These results also suggest the radical property of the process. In order to identify the hydrogen atom source of the products, we then carried out several deuteriumlabeling experiments with substrates 1a-I and 1a-I-D2. The reaction of 1a-I-D2 and 2b under the standard conditions did not result in any incorporation of deuterium into the product 3ab (Scheme 4B, eq 2); in contrast, the model reaction of 1a-I and 2b in acetone-d6 as solvent led to a significant amount of deuterated product (Scheme 4B, eq 3). Notably, we have also conducted the following experiments by treatment of product 3ab with acetone-d6 or a mixture of D2O/2-butanone under irradiation in the dark. However, 3ab did not undergo any hydrogen/deuterium exchange, ruling out the possibility of hydrogen/deuterium exchange between the product and solvent.20 These outcomes imply that the reaction media should serve as a hydrogen atom donor. Moreover, we aslo performed the intermolecular competitive and parallel experiments with 1aI and 1a-I-D2 by reacting with 2e under the standard conditions (Scheme 4C). The kinetic isotope effect (KIE) was calculated based on the formation of aldehydes 6, 6-D and product 3ae. The KIE values were determined to be 2.13 and 2.18, respectively (eqs 4−6), suggesting that HAT-based activation of oxime ethers by abstraction of the benzylic H-atom should play an important role in the reaction. At the current stage, the

a

Conditions: 1 (0.2 mmol), 2b (0.4 mmol, 2.0 equiv), 2-Cl-AQN (0.5 equiv), K2CO3 (0.4 mmol, 2.0 equiv), and 2-butanone (4.0 mL) under the irradiation of 370 nm LEDs. bIsolated yield. cThe d.r. was determined by HPLC analysis using a chiral stationary phase.

producing the corresponding ketonitriles 3bb−ib in 67−85% yields. Furthermore, 3,3-disubstituted O-benzyl oxime ethers 1j−l and even sterically hindered substrates such as 1m also participated in the reaction smoothly to deliver products 3jb− mb with 63−67% yields. Oxetan-3-one derived oxime ether 1n also proved to be suitable for the reaction with product 3nb being isolated in 81% yield. Notably, the reaction could be successfully extended to bicyclo[3.2.0]hept-3-en-6-one dervied substrate 1o, and the expected product 3ob was obtained in 73% yield and moderate dr due to the radical property of the process. Remarkably, the reaction of less strained, five-membered 2substituted cyclopentanone (1p) and 2-methyltetrahydrofuran3-one derived substrates (1q) also worked well, furnishing products 3pb and 3qb in 63% and 57% yields, respectively. Next, we turned our attention to evaluation of the substrate scope of alkenes 2 by reacting them with 1h under the standard conditions (Scheme 3). Various 2-methylacrylates with different substituents at the ester moiety showed good reactivities, and the corresponding products 3hc−he were obtained in 64−73% yield. Moreover, both 4-methylphenyl vinyl ketone (2f) and phenyl 1-phenylvinyl ketone (2h) also proved to be competent radical acceptors, giving 3hf and 3hh in 63−79% yields. In addition, ethyl vinyl ketone 2i reacted well with 1h to afford product 4hi in 59% yield. N-Phenyl-2-(p-tolyl)acrylamide 2j could also serve as a suitable radical acceptor to furnish 3hj in 61% yield. Ally sulfones have recently been extensviely used as radical acceptors to install an allyl group.21 Remarkably, our C

DOI: 10.1021/acs.orglett.9b02535 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 4. Mechanistic Studies

Scheme 5. Plausible Reaction Mechanism

radical addition across alkene 2b and another HAT from the solvent 2-butanone (BDFE ≈ 91 kcal mol−1) to produce the final product 3ab (BDFE ≈ 96−98 kcal mol−1).23 An HAT event betwen the 2-butanone radical and 2-Cl-AQN-H· occurs to regenerate the ground state catalyst 2-Cl-AQN and 2butanone, thereby closing the catalytic cycle. Ethynylbenziodoxolones (EBX) reagents have recently been established as a powerful alkynylation reagent of radicals by our group and others.24 To further demonstrate the utility of benzyl cycloketone oxime ethers, we preliminarily attempted the alkynylation of cyanoalkyl radicals by reacting 1a-III with PhEBX 7 using 2,3-butadione as photocatalyst (Scheme 6, eq 7). Under the slightly modified conditions, the desired alkynylation product 8 was isolated in 68% yield. Scheme 6. Radical C−C Bond Cleavage/Alkynylation Cascade of Benzyl Cycloketone Oxime Ethers

generation of a benzylic radical via proton concerted electron transfer (PCET) pathway could not be ruled out.22 However, during optimizing studies, we found that substrates 1a-I−IV demonstrated obviously different activities and reaction efficiency, though their oxidation potential range (+1.37− +1.86 V vs SCE) suggested that PCET between all of those substates and photoexcited state 1-Cl-AQN*(+1.9 V vs FC/ FC+) should be favorable. Moreover, light on−off experiments revealed that the desied reaction proceeded smoothly upon light irradiation, but no further progress was observed when the light source was removed. These results suggest that radical chain process should not be the main mechanistic component.20 Building on the above results and our previous studies,11,13 we propose a plausible mechanism as outlined in Scheme 5. The reaction begins with an abstraction of the hydrogen atom at the bezylic position of 1a-I by the photoexcited catalyst 2-Cl-AQN* to give benzylic radical 1a-I′ and 2-Cl-AQN-H·. An alternative pathway toward the formation of benzylic radical 1a-I′ might also be possible, which involves a sequential SET of the electronrich phenyl ring by 2-Cl-AQN* and a deprotonation process. The mechanistic studies suggest that the HAT process should be the major pathway. Then, benzylic radical 1a-I′ undergoes a facile β−N-O bond scission to afford cyclic iminyl radical 1a-I′′ with release of p-anisaldehyde 6. During the optimization study (Table 1, entry 16), we have indeed isolated the aldehyde 6 in 94% yield. Cyclic iminyl radical 1a-I′′ undergoes a further β-C− C bond cleavage to form cyanoalkyl radical 1a-A, followed by a

In conlusion, we have developed a light-driven, metal-free, and iminyl radical mediated ring-opening C−C bond cleavage/ addition cascade of O-4-methoxybenzyl oxime ethers and alkenes. This chemistry exhibits good substrate scope and functional group tolerance with respect to both components, providing an efficient and complementary method for the synthesis of various valuable oxo nitriles.25 In contrast to the previous methods for iminyl radical generation, the current strategy opens a new way for reaction design of cyclic iminyl radicals.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b02535. Experimental procedures, full analysis data for new compounds, and copies of NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. D

DOI: 10.1021/acs.orglett.9b02535 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters ORCID

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Jia-Rong Chen: 0000-0001-6054-2547 Wen-Jing Xiao: 0000-0002-9318-6021 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the NNSFC (21622201, 91856119, 21820102003, and 21772053), the Science and Technology Department of Hubei Province (2017AHB047), the Fundamental Research Funds for the Central Universities (CCNU19TS051), and the Program of Introducing Talents of Discipline to Universities of China (111 Program, B17019) for support of this research.

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DOI: 10.1021/acs.orglett.9b02535 Org. Lett. XXXX, XXX, XXX−XXX